In: Wagner W., Szekely, B. (eds.): ISPRS TC VII Symposium - 100 Years ISPRS, Vienna, Austria, July 5-7, 2010, IAPRS, Vol. XXXVIII, Part 7B 
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merely the displacements obtained at the previous described 
step. When more than one tile is considered, then a set of 
candidates for 5 X and 5 y are obtained, which inevitably may 
contain some misleading values. Therefore, an outlier removal 
stage is required, in order to consider only those correct shifts 
and achieve acceptable (subpixel) accuracy. The estimation of 
8 X and 8 y consisted on a statistical procedure based on the 3D 
histogram obtained from 8 X and 8 y . 
3. RESULTS 
3.1 Dataset 
The proposed methodology was applied to three pairs of 
images, comprising medium and high (urban and urban/rural 
context) spatial resolution images (Figure 3). The digital 
elevation model (DEM) of the considered regions obtained from 
the shuttle radar topography mission (SRTM) (Farr, 2004) is 
presented in Figure 4. 
Figure 3. Reference images (left) and new images to be 
registered (right), regarding a medium spatial resolution pair 
(first row, Landsat/ASTER), a high spatial resolution pair with 
urban/rural context (second row, Orthophoto/IKONOS) and a 
high spatial resolution pair with rural context (third row, 
Orthophoto/ALOS). Further details in subsection 3.1. 
3.1.1 Medium spatial resolution 
The first pair of images consists in two segments with 512x512 
pixels: one obtained from an orthorectified panchromatic 
Landsat image; and the other from an ASTER image (NIR 
band) with an approximate geometric correction. Both of these 
images are from the northwest of Portugal. Both images have a 
pixel size of 14.25m and present a temporal difference of 1.5 
years. The two segments are represented in Figure 3 (first row). 
y(Row) 0 0 x (Pixel) 
Figure 4. DEM (based on the SRTM) associated to the 
Landsat/ASTER pair (upper left), orthophoto/IKONOS pair 
(upper right) and Orthophoto/ALOS pair (bottom). Further 
details in subsection 3.1. 
As can be observed in Figure 4, the considered region for this 
pair of images presents a considerable terrain height variation, 
ranging from 29m to 487m (according to the SRTM DEM). 
However, the transformation function defined in (1) and (2) 
may be considered adequate due to the ASTER acquisition 
geometry. The reference shifts were manually obtained through 
the identification of 4 conjugate points, associated to an 
average+standard-deviation of 2.1+0.1 and 1.4±1.3 pixels, for 
the horizontal and vertical directions, respectively. The low 
standard-deviation of the manually identified conjugate points 
support the adequacy of the transformation function. 
3.1.2 High spatial resolution (IKONOS): urban/rural context 
The second pair of images is composed by two segments with 
512x512 pixels: one corresponding to the green band of an 
orthophoto; and the other corresponding to a panchromatic 
IKONOS image with an approximate geometric correction. 
These images cover a small part of the city of Porto (Portugal) 
with a mixture of urban with rural context, have a pixel size of 
lm and present a temporal difference of around 1 year. The two 
segments are represented in Figure 3 (second row). 
As can be observed in Figure 4, the considered region for this 
pair of images presents a smooth terrain elevation, ranging from 
67m to 87m (according to the SRTM DEM). The reference 
shifts were manually obtained through the identification of 4 
conjugate points, associated to an average+standard-deviation 
of 15.9+1.8 and 5.7+0.2 pixels, for the horizontal and vertical 
directions, respectively. 
3.1.3 High spatial resolution (ALOS): rural context 
The third pair of images comprises two segments with 512x512 
pixels: one corresponding to the NIR band of an orthophoto; 
and the other corresponding to a panchromatic ALOS-PRISM 
image with an approximate geometric correction. These images 
are from the centre of Portugal (rural context), have a pixel size